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USB Attached SCSI Protocol (UASP) Curtis E USB Attached SCSI Protocol (UASP) Curtis E. Stevens Sr. Staff Engineer Standards & Specifications Western Digital Technologies, Inc. AgendaAgenda • Justification for New Protocol Development • UAS Overview • Device Discovery • SCSI Support 2 New Protocol Goals & Justification WhyWhy AA NewNew ProtocolProtocol • USB 1.1 completed in 1998 • BOT (Bulk-Only Transport) completed development in 1999, over 10 years ago • Designed to support USB 1.1 • USB throughput was approximately the same as a parallel port • Mass storage devices were mainly Flash dongles, Floppy drives, and CD-ROM drives • Speed/capacity ratios remained low • Devices only needed to receive and process one command at a time 4 WhyWhy AA NewNew ProtocolProtocol (continued)(continued) • USB 2.0 completed in 2000 • BOT not updated • Speed/capacity ratio allowed larger devices including hard disk to become acceptable • Bus speed was still slower than many devices making command queuing not necessary • Attached devices continue to speed up and the USB bus remains a limitation • USB 3.0 completed in 2008 • Speed/capacity ratio now allows much larger devices including RAID • The USB bus no longer is slower than the attached device • BOT protocol has system challenges which limit full bus utilization New protocol is needed 5 UASUAS DevelopmentDevelopment GoalsGoals • Address system limitations due to protocol overhead • Enable maximum asynchronous processing • Increase the efficiency of devices • Allow queuing • Enable Task Management to support management of many commands • Enable device command and control capabilities using today’s standards • SPC-4, SBC-3, and SAM-5 • Existing performance driver stacks 6 Bulk-OnlyBulk-Only TransportTransport CommandCommand AnalysisAnalysis 7 BOTBOT LimitationsLimitations • Round trip times in non-queuing drivers • Time from command completion to receiving the next command can be longer than the actual command • Round trips between BOT driver and USB driver • There are normally 3 round trips between the BOT driver and the USB driver for each command • Each round trip is one or more USB bus transactions • Each round trip has to completed before the next one begins • Bus utilization is lost when the USB driver completes a round trip and is waiting for the next 8 UASUAS CapabilitiesCapabilities –– USB USB 2.02.0 • Provides a modest increase in bus utilization • Provides a mechanism for the device to store several commands and optimize the use of its media • Complies with current command and transport standards • Eliminates much of the round trip time between command phases • Hidden behind queuing • Switches the storage driver to a queuing model 9 UASUAS CapabilitiesCapabilities –– USB USB 3.03.0 • Maximizes bus utilization through the use of streams • Provides a mechanism for the device to store several commands and optimize the use of its media • Complies with current command and transport standards • Eliminates much of the round trip time between command phases • Hidden behind queuing • Switches the storage driver to a queuing model 10 USB Attached SCSI Overview ResourcesResources • 4 pipe model • Command, Status, Data-in, Data-out • Protocol does not mix commands, data, or status • Eliminates the need to wait or for the next command phase • Pipe Usage Descriptor • BOT relied on pipe type for usage • UAS has several pipes with an explicit declaration about the usage of each pipe • Allows for vendor specific pipes to be present • This case not well handled by BOT 12 TypicalTypical CommandCommand SequenceSequence –– USB USB 2.02.0 • Setup Status pipe to receive status • Issue commands on the Command pipe • Several commands can be issued • When status is received • Re-setup the Status pipe with a new buffer • If the packet is a SENSE IU, then complete the command with the associated tag • If the packet is a READ READY IU, then setup the DATA- IN pipe to transfer data from the device to the host • If the packet is a WRITE READY IU, then setup the DATA- OUT pipe to transfer data from the host to the device 13 TypicalTypical CommandCommand SequenceSequence –– USB USB 3.03.0 ---- For For eacheach commandcommand ---- • Assign a stream ID • Setup a status buffer on the Status pipe with the associated stream ID • If the command reads data, then setup a read buffer on the DATA-IN pipe with the associated stream ID • If the command writes data, then setup a write buffer on the DATA-OUT pipe with the associated stream ID • Send the command with the stream ID as the command tag • The Command pipe is NOT a streams pipe 14 TypicalTypical CommandCommand SequenceSequence –– USB USB 3.03.0 ---- When When statusstatus isis receivedreceived ---- • The stream ID indicates the command that has completed • If the command required data transfer, then ensure that the transfer has completed • If the transfer is not completed, then either the data is still in flight (e.g., the USB driver has received the data but not completed the outstanding transaction to the UAS driver) • Or, there is a data underrun • In this case, the UAS driver reports a service delivery subsystem error to the layer above • Report command completion to the level above the UAS driver 15 UsingUsing SCSISCSI onon USBUSB • Commands are sent to the device without waiting for data transfers or individual statuses • The device will return a queue full on the status pipe when it is no longer able to accept commands • The device selects commands to process • The host can set command priority • The device drives the usage of the Status, Data-in, and Data-out pipes • The host may abort individual commands in the device command queue 16 UsingUsing SCSISCSI onon USBUSB (continued)(continued) • The host needs to be sure a clean up loose transactions • Although UAS requires the data transfer to be acknowledged as complete on the data pipe before status is reported, the asynchrony of the UAS protocol may cause the UAS driver to see status reported before the data transfer is reported as complete • This happens and needs to be accounted for in the UAS driver • If the status reports an error, then the host needs to clean up loose transactions • Data transfer may still be outstanding 17 FigureFigure NotationNotation ITr (Pipe[_stream] [,IU]) – Input Transaction request OTr (Pipe[_stream] [,IU]) – Output Transaction request ITc (Pipe[_stream] [,IU]) – Input Transaction complete OTr (Pipe[_stream] [,IU]) – Output Transaction complete Pipes IUs Cmd – Command pipe CIU – COMMAND IU Stat – Status pipe SIU – SENSE IU Data-in – Data-in pipe RIU – RESPONSE IU Data-out – Data-out pipe 18 ExampleExample CommandCommand SequenceSequence (Non-data)(Non-data) 19 Non-dataNon-data TransferTransfer • Status reporting uses a stream on the status pipe • Command may be completed while other commands are transferring data 20 ExampleExample CommandCommand SequenceSequence (Write-data)(Write-data) 21 Write-dataWrite-data TransferTransfer • Uses a stream on the status pipe • Uses a stream on the data-out pipe • Command may be completed while other commands are transferring data or reporting status • Notice that there is no round trip wait for the write data transfer to begin and the status to be received 22 ExampleExample CommandCommand SequenceSequence (Read-data)(Read-data) UAS Driver xHCI Driver xHCI Controller USB-3 Device Command Device may ITr (Stat) receive these ITr (Data-in) ITr (Stat) ITr (Data In) in any order OTr (Cmd, CIU) IN ACK (Data-in,Prime) OTr (Cmd, CIU) NRDY (Data-in, Prime) IN ACK (Stat,Prime) NRDY (Stat, Prime) DP (Cmd, CIU) ACK (Cmd) OTc (Cmd) OTc (Cmd) . ERDY (Data-in) IN ACK (Data-in) DP (Data-in) IN ACK (Data.-in) . DP (Data-in) ITc (Data-in) ITc (Data-in) IN ACK (Data-in) ERDY (Stat) IN ACK (Stat) DP (Stat, SIU) ITc (Stat, SIU) ITc (Stat, SIU) ACK (Stat) Status Key: Status Pipe, Command Pipe, Data-in Pipe, UAS Command Sequence 23 Write-dataWrite-data TransferTransfer • Uses a stream on the status pipe • Uses a stream on the data-in pipe • Command may be completed while other commands are transferring data or reporting status • Notice that there is no round trip wait for the read data transfer to begin and the status to be received 24 ExampleExample CommandCommand SequenceSequence (Abort(Abort Command)Command) 25 AbortAbort CommandCommand • Uses a stream on the status pipe • Abort command may be completed while other commands are transferring data or reporting status • Abort task should be followed by a status on the status pipe for the command being aborted 26 ExampleExample CommandCommand SequenceSequence (Multiple(Multiple Command)Command) 27 MultipleMultiple CommandCommand • Both commands are processed at the same time • Data for the second command starts transfer before the data for the first command • There is a period of time where data is transferring in both directions at the same time • High level of USB bus utilization • Once the first command is received, the only time delays are waiting for the device • Notice the high level on asynchrony on the USB bus • The device chooses to complete the first command received before the second command received • The device could have chosen to complete the second command first 28 UAS Device Discovery UASUAS DescriptorsDescriptors • Configuration Descriptor – Follows USB specifications • Device Descriptor – Follows USB specifications • Interface Descriptor – Follows USB specifications • For backward compatibility – BOT interface should be 1 st and UAS interface should be 2 nd • If backward compatibility is not important, then only UAS interface is necessary
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